The present invention relates to an apparatus and method for tracking the direction of a user""s gaze. The invention has particular relevance to an eye tracking system for use with optical instruments which form a viewable image of an object, such as microscopes, cameras, telescopes etc.
The applicant has already proposed in the PCT patent application PCT/EP 95/04178 an in the publication WO96/13743 a microscope system which employs eye tracking techniques to track the position of gaze of a user viewing an image through a microscope eyepiece, which gaze information is used to controls for example, an auto-focus system. This is particularly useful at high magnification, where the depth of field is often limited and only a small part of the total field of view is in sharp focus at any one time. This earlier patent also teaches that the gaze information can be used to control other function of the microscope, including hands-free movement of the microscope or the operation of a menu-driven computer system superimposed on the user""s normal field of view. for eye tracking. The two most common of these are Limbus trackers and video eye trackers.
Limbus trackers usually operate by illuminating the user""s, eye, typically with one or more infra-red LEDs and detecting the light reflected off the white of the eye (sclera) using one or more photodetectors. Since the amount of light reflected off the white of the eye will vary depending on the position of the dark regions (the pupil and the iris), it is possible to determine where in the specified field of view the user is actually looking. However, this type of eye tracking system cannot unambiguously determine the angle of gaze because it only gives information relating to the position of the iris-sclera boundary. In addition, whilst Limbus tracking techniques give fairly good information on the horizontal position of the surface of the eye, they cannot accurately determine the vertical position due to obstruction from eyelashes and eyelids.
There are many different video-based eye tracking techniques. Some of these simply illuminate the eye and view the pupil using an imaging system. By determining the centre of the pupil from the image, information relating to the pointing direction of the eye can be obtained. This technique, however, suffers from the problem that movements of the observer""s head cannot be distinguished from movements of the eye.
Other more sophisticated video-based eye tracking systems have been proposed which detect the position of the Purkinje images, which are the reflections of the illumination source off the surfaces of the cornea and the lens (often referred to as the highlights or glints) Whilst this technique is relatively accurate because it is independent of head movements, it suffers from the problem that some of the Purkinje images are extremely weak and therefore difficult to image clearly.
One aim of the present invention is to provide a different, accurate eye tracking technique for use with optical instruments which form a viewable image of an object, such as microscopes, cameras and the like.
According to one aspect, the present invention provides a new eye tracking system instrument in general and an optical instrument for forming a viewable image of an object comprising: an objective lens for forming a viewable image of the object at an image plane; an eye sensor for sensing a direction of gaze of a user viewing the viewable image; and means for controlling a controllable function of the optical instrument in dependence upon the sensed direction of gaze; characterised in that the eye sensor comprises: (i) a sensor lens for focusing light reflected from the retina of the user; (ii) an imaging transducer located at a plane which, when the user views said viewable image, is commonly conjugate with said image plane to the retina of the user""s eye, for generating an electrical image signal of the portion of the user""s retina which can be seen through said sensor lens; (iii) a memory for storing retinal information; and (iv) comparing means for comparing signals representative of the retinal image signal generated by said imaging transducer with said stored retinal information to generate gaze information indicative of the direction of gaze of the user. This system has the advantage that the direction of gaze determined by the eye sensor is independent of any movement of the user""s head.
Preferably, one or more eyepieces are provided to facilitate the viewing of said viewable image. In a preferred form of the instrument, an illumination source is provided for illuminating the retina of the user""s eye in order to improve the quality of the retinal image generated by the imaging transducer. Preferably the illumination source has a number of light emitting portions which are arranged off the optical axis so that selected ones of the light emitting portions can be illuminated at any one time in order to avoid the effects of cornea glints.